1. Field of the Invention
This invention relates to improvements in methods and circuits for detecting and distinguishing problems in the operation of an automotive alternator, and more particularly to distinguishing between a broken stator phase signal wire and a broken alternator belt.
2. Technical Background
In modern cars and trucks, the battery charging systems typically include a three phase synchronous alternator to produce an ac output voltage. A bridge circuit rectifies the ac voltage, and a voltage regulator maintains the voltage at a desired level.
The two basic parts of the alternator are the rotor and the stator. The rotor has a dc-excited winding, called the "field coil," to set up a rotating magnetic field. The stator has a three-phase winding that generates an ac emf to provide an output voltage. A belt connected to the engine turns the rotor, and the magnetic flux moving across the stator coils produces the ac emf on the stator. The amount of current generated is proportional to the speed of the rotor and the average current in the field coils. Typically a full-bridge rectifier having six diodes rectifies the generated ac voltage to charge the battery and supply energy for electrical loads.
The regulator is a circuit designed to provide a constant system output voltage to charge properly the battery to avoid damaging it. The system also provides a stable power supply to electrical loads. The regulator compensates for variations in the system loading, including battery charge levels, and for variations in rotor speed produced by changes in engine speed. The regulator performs this compensation by altering the average current through the rotor field coils and changing the dc level of the output voltage on the stator coils.
Until recently, the regulator consisted of several discrete electronic components. Then, with the arrival of integrated circuit technology, the automotive industry created a regulator that included a power transistor, some filtering capacitors, and a basic control circuit integrated onto a single integrated circuit device. In addition, the devices began to exhibit more sophisticated means for voltage regulation, fault-detection, and self protection on one integrated circuit device.
A frequent problem that may occur is the breakage of the belt that drives the alternator rotor, so that the battery is no longer charged. It is important to detect rapidly this kind of breakdown to avoid continuously supplying current to the field coils in the rotor and needlessly running down the battery. That is why in integrated regulators there is typically a "phase sequence" signal derived from a stator winding. This signal can be used also to detect whether the alternator is charging normally. When this sequence is lacking, it means that, for some reason, the alternator is not functioning normally.
Still, sometimes the wire that transmits the phase sequence information from the alternator to the regulator chip breaks. Since the regulator cannot distinguish between the alternator belt breaking and the phase sequence wire opening, the regulator stops the flow of current to the winding in the rotor and the charging system from charging the battery. Thus, this occurs though the system itself has not really undergone any breakdown. At first, a broken phase sequence wire was considered of little importance, because it occurs infrequently compared to a broken alternator belt. Thus, systems were designed to stop the flow of current to the rotor field coils every time the phase sequence was missing.
Now, it is desirable to distinguish between the case of the broken alternator belt and a broken phase sequence wire. A broken alternator belt justifies a system shutdown, but a broken phase sequence wire does not. Thus, the system can run more intelligently.
What is needed is a method and apparatus for making such a distinction.